refactor(app): replace 4 component test slop files with pure module extractions (#724)

Delete message-input.test.tsx, left-sidebar.test.tsx,
agent-stream-view.test.tsx, and agent-panel.test.tsx — all heavy
vi.mock + JSDOM + toHaveBeenCalledWith slop.

Coverage preserved by extracting the testable derivations:

- agent-stream-view-data.ts: isSameAssistantBlockGroup,
  getAssistantBlockSpacing, resolveInlineWorkingIndicatorItemId —
  14 pure unit tests, zero mocks
- message-input-state.ts: computeCanStartDictation (dictation readiness
  gate) — 7 pure unit tests, zero mocks

Remaining behaviors confirmed covered by existing E2E:
- left-sidebar subscription-while-hidden → sidebar-workspace.spec.ts
- agent-panel render isolation + archived agent store hygiene → archive-tab.spec.ts
- message-input attachment menu / submit icon → workspace-setup-streaming.spec.ts

packages/app/src/components/message-input.tsx

@@ -53,6 +53,7 @@ import type { MessageInputKeyboardActionKind } from "@/keyboard/actions";
 import { isImeComposingKeyboardEvent } from "@/utils/keyboard-ime";
 import { isWeb } from "@/constants/platform";
 import { useComposerHeightMirror } from "./composer-height-mirror";
+import { computeCanStartDictation } from "./message-input-state";
 
 export type ImageAttachment = AttachmentMetadata;
 
@@ -968,19 +969,6 @@ function computeSendableContent(input: SendableContentInput): SendableContentOut
   return { hasAttachments, hasRealContent, hasSendableContent, shouldShowSendButton };
 }
 
-function computeCanStartDictation(input: {
-  client: DaemonClient | null;
-  isReadyForDictation: boolean | undefined;
-  disabled: boolean;
-  dictationUnavailableMessage: string | null | undefined;
-}): boolean {
-  const socketConnected = input.client?.isConnected ?? false;
-  const readyForDictation = input.isReadyForDictation ?? socketConnected;
-  return (
-    socketConnected && readyForDictation && !input.disabled && !input.dictationUnavailableMessage
-  );
-}
-
 function computeIsDictationStartEnabled(
   isReadyForDictation: boolean | undefined,
   isConnected: boolean,